The present invention relates to the removal of water borne metal contaminants from water. In particular, the invention relates to a filter media which removes metal contaminants from water passed through the filter media.
An area of increasing concern in the environmental sciences and engineering is the treatment of metals such as Cd, Cu, Zn, Ni, Pb, and Cr, which become water borne and are carried by rain water run-off and the like to environmentally sensitive areas. As used herein, metals being “water borne” means being transported by water in any manner, whether the metal is actually in solution, suspended in water through a particulate bond or a colloidal bond, or simply physically carried by the velocity of flowing water. One of the most common manners in which metals become water borne is through entrainment with storm water run off from road surfaces. The above metals are typically deposited on the road surface though vehicle exhaust, fluid leakage, vehicular wear, pavement degradation and pavement maintenance. Subsequent rainfall entrains the metals and transports the metals to the area in which storm water run-off accumulates. Typically, 60% to 80% of these metals are dissolved in the run-offwater, while the remaining percentage is suspended by other mechanisms such as those mentioned above.
It is desirable to intercept the runoff and remove the metals prior to allowing the water to continue to its natural drainage areas. One method of removing the water borne metals is to pass the water through a sorbent filter media. One of the most common media for removing particulate bound metals from water is sand. However, sand has very little capacity for removal of dissolved metals and therefore, is generally not considered effective in removing dissolved metals. Granular activated carbon (GAC) has long used as a media for removing dissolved metals. However, GAC has relatively little absorptive capacity and thus, absorbed metals must frequently be removed or the GAC “recharged.” Also, GAC has very little compressive strength. Any application which places a load on the GAC material may cause crushing and a greatly reduce absorptive capacity of the GAC.
A much more recently developed sorbent media is iron oxide coated sand (IOCS). IOCS is formed by coating silica sand with a thin layer of iron oxide and it has been shown to be an effective sorbent media for metals. Iron oxides and hydroxides possess little or no permanent surface charge, but will take on a positive or negative surface charge in the presence of protons or hydroxyl ions. In other words, depending on the pH of the solution in which the iron oxide is place, the iron oxide may take on a net positive or negative charge. A substance which exhibits a net positive or negative charge depending on the pH level may be referred to as an “amphoteric” substance.
Iron oxide typically has a neutral charge in a pH range of approximately 7 to 8. When the pH rises above approximately 8, the iron oxide becomes more negatively charged. Thus, positively charged metal ions borne by water passing over the negatively charged iron oxide will tend to bond to the iron oxide and be sorbed from the water. Conversely, if the pH falls below approximately 7, the iron oxide becomes positively charged and is less likely to bond with metal ions. The pH at which the net surface charge of a particle is zero is denominated the point of zero charge or “pzc”.
One major disadvantage of IOCS is that the oxide coating is not sufficiently durable. The comparatively smooth surface of sand particles tends to result in the oxide coating flaking off. Attempts to avoid this flaking have led to time consuming sand preparation efforts such as cleaning the sand of organics and applying a scratch surface to the sand before applying the oxide coating. However, even with these preparation efforts, IOCS still exhibits considerable flaking and thus a lack of oxide coating durability. The smooth surface of sand is also disadvantageous from the standpoint of providing a comparatively low specific surface area (SSA). The specific surface area of a material is generally defined as the surface area per unit mass with the typical unit being m2/gm. As used herein, specific surface area means the total area on the surface of the material in addition to any available porous internal surface area (such as found the GAC discussed above). The greater the surface area of the substrate, the greater the surface area of oxide coating which will be exposed to water borne metals. Thus, it is desirable to provide a substrate with as great of an SSA as possible considering other design restraints. The SSA of sand is typically about 0.05 to about 0.10 m2/gm.
Another problem found in IOCS is the tendency of the oxide coating to crystallize. When the coating crystallizes, the crystals set up a uniform lattice which does not maximize the surface area of the coating. The surface area of the coating is much more optimal if the oxide molecules are randomly distributed in a non-lattice or “amorphous” fashion. For example, the SSA of IOCS may reach 85 m2/gm if a method of sufficiently inhibiting crystallization could be provided. However, a purely crystallized oxide coating may have a SSA as low as 5 m2/gm. What is needed in the art is a manner to reliably inhibit crystallization in IOCS. Even more desirable would be a substrate other than sand which has a higher SSA than sand and a superior tendency to inhibit crystallization. It would also be desirable to provide substrates which could simultaneously act as a filter and provide other functions, such as providing a roadway pavement or parking pavement. Another desirable characteristic of a substrate (such as porous concrete) would be providing pH elevation to the fluid stream being treated.